ATAG 154 Fault Code: Causes, Fixes & Repair Costs
What does the ATAG 154 fault code mean?
ATAG fault code 154 indicates that the boiler has detected an abnormal temperature condition: the return water temperature is higher than the flow temperature, and the flow temperature itself is rising too rapidly — by somewhere between 7°C and 15°C within a single second. This suggests that water is not circulating properly through the system, causing heat to build up quickly at the heat exchanger rather than being distributed around the heating circuit. In plain terms, the boiler is detecting a sharp drop in water flow, which can be an early sign of overheating. The fault will typically lock the boiler out as a safety precaution.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Faulty or intermittently sticking circulation pump Common
The most common culprit. If the pump is not spinning freely or is running too slowly, water cannot circulate fast enough around the heating circuit. This causes heat to concentrate at the heat exchanger, making the flow temperature spike rapidly — exactly what triggers code 154. Pumps can seize partially after a period of inactivity, particularly at the start of the heating season.
- Closed TRVs with no adequate bypass Common
When thermostatic radiator valves close down — especially if all or most radiators have TRVs fitted — the system flow resistance increases dramatically. If there is no bypass valve, or it is undersized, the pump cannot push water around the circuit and temperatures rise sharply at the boiler. ATAG's own guidance for models such as the iR18 states that a 22mm automatic bypass valve is mandatory where an external pump is fitted.
- Air trapped in the system Sometimes
Air locks in the pipework or heat exchanger can restrict water flow, causing localised hot spots and rapid temperature changes. This is especially common after recent system work, a power cut, or at the start of a new heating season when the system has been dormant.
- Blocked system filter or closed isolation valve Sometimes
A magnetite or debris-laden system filter (such as a Magnaclean or similar) that has not been serviced recently can restrict flow to the point where the boiler cannot circulate water adequately. Equally, a partially closed isolation valve on the flow or return pipework can mimic the same effect.
- Flow switch wiring fault or faulty flow switch Rare
If the wiring harness between the PCB and the flow switch (typically connector CN12, pins 9 and 10 on ATAG i-series boards) has a loose connection, corrosion, or a break, the boiler may misread the flow conditions and display a spurious 154 fault even when the pump itself is working correctly.
How to fix it
- Reset the boiler and observe whether the fault returns quickly DIY safe
Press and hold the reset button as described in your ATAG user manual. If the boiler fires and runs normally for an extended period, the fault may have been a transient blip. If it returns within minutes or after one or two heating cycles, there is an underlying problem that needs investigating. Do not reset more than two or three times in quick succession.
- Check that all radiator valves and isolation valves are open DIY safe
Walk around your home and make sure that every thermostatic radiator valve (TRV) is turned up or fully open, and that none of the lockshield valves on radiators are closed off. Also confirm that the isolation valves on the boiler's flow and return pipes are both fully open. Restricting flow at any of these points can trigger the fault.
- Check and top up system pressure if low DIY safe
Look at the pressure gauge on the boiler. For most ATAG boilers the cold system pressure should sit between 1.0 and 1.5 bar. If it reads below 1.0 bar, use the filling loop to top it up to around 1.2–1.5 bar, then reset. Low pressure can reduce circulation and contribute to flow imbalance.
- Bleed radiators to remove trapped air DIY safe
If you hear gurgling from radiators or they feel cool at the top, use a radiator bleed key to release trapped air from each radiator, starting with the ones furthest from the boiler. Once water flows steadily, close the bleed valve. Check system pressure again afterwards and top up if needed, as bleeding can cause pressure to drop.
- Check that the system filter has been cleaned recently DIY safe
If your system has an inline filter (such as a Magnaclean or Fernox TF1), check when it was last serviced. A heavily blocked filter is a very common cause of restricted flow. If it is due for cleaning, a Gas Safe engineer can do this during the same visit.
- Have a Gas Safe engineer inspect and test the circulation pump Gas Safe engineer
An engineer will check that the pump is powering up correctly, spinning freely, and running at the right speed setting for the system size. A pump that is intermittently seizing — sometimes running, sometimes not — is the most frequent cause of code 154 on ATAG boilers. The engineer may try freeing a stuck pump first; if it cannot be reliably restored, replacement will be recommended.
- Have the bypass valve checked or installed Gas Safe engineer
An engineer will confirm whether a suitably sized automatic bypass valve is fitted and functioning correctly. On ATAG models including the iR18, a 22mm automatic bypass is required where TRVs are fitted throughout the system. A missing or undersized bypass prevents adequate flow when TRVs close down, and is a common oversight in older installations.
- Have the flow switch wiring and connections tested Gas Safe engineer
An engineer with appropriate test equipment can check continuity across the flow switch wiring harness at the PCB (connector CN12, pins 9 and 10 on i-series models) to rule out a wiring or connector fault producing a false code 154. Corroded or loose pins can be cleaned or replaced without necessarily needing a new PCB.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If the above homeowner checks have not resolved the issue, or if you are unsure at any stage, contact a Gas Safe registered engineer. Code 154 relates to hydraulic flow and temperature sensing — work on the pump, bypass, pipework, and internal wiring must be carried out by a qualified professional. Do not attempt to open the boiler casing or interfere with internal components yourself.
Parts you may need
- Central heating circulation pump (e.g. Grundfos UPS2 or equivalent) · from £95
- 22mm automatic bypass valve · from £35
- Inline system filter (e.g. Magnaclean Pro2 or Fernox TF1) · from £55
- Flow switch assembly (ATAG compatible) · from £45
- Wiring harness / PCB connector repair kit · from £20
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £150–£380, depending on the underlying cause.
Frequently asked questions
Is ATAG fault code 154 dangerous?
Not immediately, but you should not ignore it. The boiler locks out as a safety precaution to prevent overheating, which is the system working as intended. However, if the underlying cause is a failing pump or severely restricted flow, leaving it unattended and repeatedly resetting without a proper diagnosis could eventually cause damage to the heat exchanger. Get it looked at by a Gas Safe engineer within a day or two rather than waiting weeks.
Can I fix ATAG fault code 154 myself?
There are a handful of checks a homeowner can safely carry out: resetting the boiler, opening up TRVs, topping up system pressure, and bleeding radiators. These occasionally resolve the fault if it was caused by a temporary air lock or a pressure drop. However, the most common cause — a faulty or sticking circulation pump — requires a Gas Safe registered engineer to diagnose and replace. Do not attempt to open the boiler casing or work on internal components.
How much does it cost to fix an ATAG 154 fault code in the UK?
For most people the repair involves a pump inspection or replacement, costing roughly £150–£380 including parts and labour. A diagnostic visit alone typically costs £85–£150. If the pump simply needs freeing rather than replacing, costs will be at the lower end. If additional work is needed — such as fitting a bypass valve or cleaning a blocked system filter at the same time — expect to pay towards the top of that range. In London and the south east, labour rates tend to be £80–£120 higher than the national average. In rare cases where a PCB fault is causing a false reading, PCB replacement can cost £300–£600, but this is uncommon for this particular fault code.
Why does my ATAG boiler keep showing fault 154 at the start of the heating season?
This is a classic sign of a pump that has seized or partially seized during the summer months when it sat unused. Circulation pumps can stick when the impeller corrodes slightly or debris settles around it during a long idle period. An engineer can often free a mildly stuck pump without replacing it, but if it continues to stick intermittently throughout the season, replacement is the more reliable long-term fix. Running the pump briefly in summer — something some ATAG controls allow — can help prevent this happening year after year.